Physical properties-durability relationships of alkali-activated volcanic ash and basalt sawing sludge pastes

Two sets of alkali-activated pastes based on natural and industrial volcanic residues from Mt. Etna (Italy), namely volcanic ash and basalt sawing sludge, were synthesized to evaluate their microstructural and durability performance for construction and restoration applications. For each precursor, standard and modified formulations were produced, with the latter incorporating Ca-bearing additives, namely slaked lime and Ca-Al bearing cement. Then, the formulations were investigated through a multidisciplinary approach including hydric behaviour, mercury intrusion porosimetry, ultrasonic pulse velocity, colorimetric analysis, infrared thermography, contact angle measurements and ageing tests. Newly obtained X-ray powder diffraction (XRPD), Vicat setting time and compressive strength data for basalt sludge-based formulations were integrated with previously reported results for volcanic ash-based binders to establish structure-property-durability relationships. Although the two volcanic precursors showed similar bulk chemical compositions, differences in particle size and mineralogical features strongly influenced the final properties of the pastes. Basalt sludge-based formulations exhibited lower water absorption and higher compactness, while the modified binder showed the highest compressive strength (i.e., 45 MPa). Conversely, the modified volcanic ash-based formulation exhibited the best freeze-thaw resistance. All samples showed high surface hydrophilicity and negligible colorimetric deviations, whereas salt crystallization represented the main durability limitation. Overall, the results highlight both the potential and the limitations of alkali-activated volcanic materials for construction and heritage conservation applications and provide useful insights for further optimization of their physical properties and durability.